1. Field of the Invention
The invention relates to a gas sensing (detection) device for detecting the concentration of a gas to be detected, an electrochemical oxygen pumping cell for electrochemically oxidizing or reducing a gas to be processed existing in the atmosphere, and a gas sensor using the same. In particular, the invention relates to a gas sensing device and an oxygen pumping cell to be used in a NOx sensor which can directly measure the concentration of NOx (nitrogen oxides) in automotive or other combustion exhaust.
2. Description of the Related Art
It has been widely demanded to measure automotive exhaust for certain gases, such as HC (hydrocarbon gas), CO (carbon monoxide), and NOx, independent of the presence of other gases. To meet such a demand, gas sensors having high gas selectivities to certain gases alone have been actively proposed in recent years.
As shown in Japanese Patent Laid-Open Publication No. Hei 9-274011, the present inventors have already proposed a mixed potential type NOx sensor of high temperature operation model that uses a zirconia solid electrolyte which is an oxygen ion conductor.
This NOx sensor has a structure in which a collector made of a precious metal such as Pt, and a NOx sensing electrode are formed on a zirconia solid electrolyte substrate, and a reference electrode (or counter electrode) is formed on the zirconia solid electrolyte at the opposite side or the same side of this sensing electrode. In this NOx sensor, a potential difference between the sensing electrode and the reference electrode can be measured to detect the concentration of NOx in the gas to be measured.
The mixed potential type NOx sensor requires that two reactions given by the following equations (1) and (2) occur on the sensing electrode at the same time. In NO2 gas detection, in contrast, reactions given by the equations (3) and (4) need to occur simultaneously. This means that NO gas detection and NO2 gas detection produce sensor outputs of opposite polarities.
In the case of detecting a total NOx concentration in automotive exhaust, coexisting NO and NO2 cause mutual interference, precluding the detection of the total NOx concentration if nothing is done. In view of this, there has been proposed a total NOx sensor of lamination type which is shown in Japanese Patent Laid-Open Publication No. Hei 9-274011.
O2+4exe2x88x92xe2x86x922O2xe2x88x92xe2x80x83xe2x80x83(1) 
2NO+2O2xe2x88x92xe2x86x922NO2+4exe2x88x92xe2x80x83xe2x80x83(2) 
2O2xe2x88x92xe2x86x92O2+4exe2x88x92xe2x80x83xe2x80x83(3) 
2NO2+4exe2x88x92xe2x86x922NO+2O2xe2x88x92xe2x80x83xe2x80x83(4) 
Here, an electrochemical oxygen pump is used to introduce oxygen from the air into the gas detection chamber so that reducing gases in the gas to be measured, such as HC and CO, are oxidized and rendered harmless. In addition, NO out of NOx is electrochemically converted into NO2, turning NOx into a single gas NO2. The total NOx concentration is detected from a potential difference based on the mixed potential of the single gas NO2.
The detection performance of such a sensor largely depends on the sensitivity characteristics and stability of the sensing electrode. Among sensing electrodes proposed heretofore are NiCr2O4 (SAE Paper No. 961130), Ptxe2x80x94Rh alloys, and thermet electrodes that contain zirconia solid electrolytes (Japanese Patent Laid-Open Publication No. Hei 11-72476). Each of these sensing electrodes has high sensitivity, whereas further improvements are required of the stability of the sensitivity. This makes it essential to secure not only the stability of the electrode materials but also the stability of a three-phase interface at which the solid electrolyte, the electrode, and the gas phase contact one another. Conventionally, it has been difficult to control the stability of the three-phase interface particularly when the metal-oxide sensing electrodes are used.
Moreover, variations in the adhesion of conventional sensing electrodes to solid electrolyte substrates have caused large property differences between the sensor devices fabricated. This sometimes caused variations in detection performance and drops in yield, contributing to unstable outputs.
Now, the detection performance of the mixed potential type NOx sensor also depends on the performance of its oxygen pump cell for converting NOx in the atmosphere into a single gas NO or NO2. That is, the higher the efficiency to convert NOx in the gas to be detected into NO or NO2, the higher the concentration range can be detected through the sensor output. Besides, the higher this conversion efficiency is, the more accurate the detection of the low concentration range becomes.
It is obvious that emission regulations become increasingly tighter worldwide in the future. The range of NOx detection in automotive engine""s exhaust gas treatment systems and the like will be shifted toward yet lower concentrations. To respond to this tide, improvements are required of the NOx conversion performance and stability of the NOx sensor described above.
To detect the concentration of NOx in exhaust with accuracy, HC, CO and other reducing gases, or interference gases, must be fully oxidized and converted into harmless gases such as H2O and CO2. Solid-catalytic oxidation and removal are insufficient for this purpose. Then, electrochemical oxidation by using an oxygen pumping cell is required. Accordingly, the performance and stability of the oxygen pumping cell for performing the oxidation of the reducing gases and the like also become important to the mixed potential type sensor.
As has been described, it is desired of recent gas sensors to measure lower ranges of gas concentrations with high accuracy and stability. It is thus an object of the present invention to improve the electrode performance of gas sensing devices and oxygen pumping cells which has a large influence on the detection performance and stability of gas sensors.
In view of the foregoing object, the present inventors propose that the object be achieved by the following means.
A gas sensing device is provided comprising: an oxygen-ion-conductive solid electrolyte substrate; a sensing electrode active to a gas to be detected and oxygen, arranged on the solid electrolyte substrate; and a reference electrode active to at least oxygen, fixed onto the solid electrolyte substrate. Here, an electrode under (base) layer made of an oxygen-ion-conductive solid electrolyte is arranged between the sensing electrode and the solid electrolyte substrate of the gas sensing device. As a result, the interface between the electrode and the solid electrolyte improves in physical and chemical adhesion. This allows stable gas detection and provides a remedy to such problems as electrode exfoliation and the occurrence of cracks at the time of fabrication.
Moreover, an oxygen pumping cell is provided comprising: an oxygen-ion-conductive solid electrolyte substrate; a first electrode active to at least a gas to be processed and oxygen, arranged on the solid electrolyte substrate; a second electrode active to at least oxygen, arranged on the solid electrolyte substrate; and means for applying a predetermined voltage to between the electrodes. Here, an electrode under layer made of an oxygen-ion-conductive solid electrolyte is arranged at least between the first electrode and the solid electrolyte substrate of the oxygen pumping cell. This also improves the physical and chemical adhesion between the electrode and the solid electrolyte, thereby reducing the interface resistance of the electrode for improved oxygen pumping performance. Furthermore, the electrode interface improves in stability, allowing a significant improvement to the degradation resistance of the sensor output.